1. Field
The present invention relates generally to infrared photodetectors. More specifically the invention relates to photodiodes of infrared detectors formed from diffusing n-type or p-type dopants into a strained layer superlattice structure to promote lateral collection of charge carriers.
2. Description of Related Art
Strained Layer Superlattice (SLS) structures have many applications in semiconductor technologies. In particular, SLS structures can function effectively as photodetectors over a wide range of wavelengths. Certain characteristics of SLS structures make them an attractive choice for photodetection. These characteristics include their relatively low Auger recombination for a given doping level, their substantially uniform properties in very long wavelength applications, and the design freedom they allow for selecting a cutoff wavelength. For these reasons, SLS structures are currently being developed for long to very long wavelength infrared (IR) detection.
In IR detector applications, SLS structures are band-gap engineered semiconductors. They are typically made by alternating thin indium-arsenide (InAs) layers with thin gallium-indium-antimonide (GaInSb) layers to form a layered structure. The quantum confinement, strain, and unusual band lineup create an anisotropic IR absorbing material from which photodiodes can be made. The photodiodes are formed by doping a region near the top surface n-type layer, etching mesas into the structure, and passivating the mesas with a coating of nitride or with a wide band-gap group III-V semiconductor.
FIG. 1 shows the architecture for a typical InAs—GaSb SLS photodetector diode, or photodiode 10. This type of photodetector can be produced with a superlattice bandgap having a cutoff wavelength that can be tuned across the infrared bands. The photodiode 10 is fabricated as a series of layers formed on a substrate in such a way to assume the general appearance of a mesa structure. The substrate, in this example, is a p-type GaSb substrate 11. A p-type GaSb buffer layer 13 is formed on the substrate 11. From bottom to top, the series of layers are: (i) a p-type SLS structure 15, (ii) an undoped SLS structure 17, (iii) an n-type SLS structure 19, and (iv) an n-type InAs layer 21. The standard photodiode fabrication procedure is to form the mesas by etching the layers to isolate the devices. The mesa of photodiode 10 is then passivated along its sidewalls with a passivation layer 23. Connection points are provided by a top Ohmic material 25 and a bottom Ohmic material 27, as shown.
The foregoing photodiode architecture suffers from a number of problems. Notably, the exposed sidewalls of the mesa (29) can be a source of undesirable excess currents that degrade the performance of the photodiode 11. In addition, the passivation process itself can be inherently difficult for an SLS detector, especially for detectors that have bare surface mesa geometries. To date, sidewall passivation techniques have not been completely effective.
Aside from the passivation layer, SLS detectors have other characteristics that inhibit the design of practical detector architecture. One problem is that SLS detectors can exhibit poor quantum efficiency (QE), i.e. the fraction of incident photons registered by the detector. This is mainly the result of poor minority carrier mobility perpendicular to the plane of the superlattice and short minority carrier lifetime. Another problem is that SLS detectors can exhibit undesirable variations in dark current and shunting effects. These variations are presumed to occur as a result of defects in the crystalline structure at the surface of the diode junction where the bandgap tends to be narrowest.